The present invention generally relates to a memory device and more particularly, to an optical memory device for recording, reproducing or erasing information by applying an optical beam such as a laser beam or the like onto a recording medium.
A magneto-optic disc memory device, one of the group of optical memory devices, is a device for recording, reproducing or erasing information by applying a laser beam onto a recording medium of a magnetic film having an axis of easy magnetization in a direction perpendicular to its surface. A method for recording or reproducing the information in the aforementioned magneto-optic disc memory device will be described hereinafter with reference to FIGS. 1, 2 and 3.
When recording, the laser beam is initially modulated in strength in accordance with a recording signal after having been converged into the laser beam having a diameter of approximately 1 .mu.m and is applied to the surface of the magnetic film, the magnetic film is locally raised in temperature. Since a portion of the magnetic film which has been raised in temperature is reduced in coercive force, when an auxiliary magnetic field is simultaneously applied thereon from outside, the information is recorded on the magnetic film through reversing the direction of the field as applied to the area to be magnetized.
When reproducing, the magnetic film on which the data been recorded on its surface is exposed to a polarized straight light of the laser having a reduced amount of light as compared to that when recording. At this moment, a polarized surface of a reflected light is inclined through a magnetc-optic effect of the magnetic film called the Kerr effect. This inclination is detected by a photo-detecting element so as to monitor the strength of the light and, upon detection of this strength of light, the information is reproduced.
Generally, in magnetc-optic memory device recording, a row of data of reproducing signals is composed of a plurality of synchronizing signal portions A each including an address portion and a sector mark indicative of a starting point of a sector, and a plurality of actual data portions B each having the information thereon. The synchronizing signal portions A and the actual data portions B are alternately positioned to form rows of data as shown in FIG. 1. The rows of data are reproduced by an information reproducing device such as shown in FIG. 2. A signal 101 is initially detected by a playback head 1 and amplified through an amplifier 2 so that a reproducing signal 102 may be outputted therefrom. The reproducing signal 102 is further processed in a waveform processing circuit 3 and led into a signal detecting circuit 4 in which the digital information signal 104 is detected. The digital information signal 104 is introduced into a PLL (Phase Locked Loop) and a synchronization detecting circuit 5. Thereafter, the PLL generates a clock signal synchronized in phase on the basis of the digital information signal 104, while the synchronization detecting circuit 5 performs a detection of synchronization in the unit of a sector such as a sector mark.
With reference to FIG. 3, a conventional method related to a timing control for recording, reproducing or erasing will be explained hereinafter.
The synchronizing signals and the actual data of the aforementioned rows of data is reproduced by the information reproducing device 7 having a construction as described above. Synchronization is detected in the synchronization detecting circuit 5 with the use of each synchronizing signal and the synchronization detecting signal 105 is led into a timing control circuit 8 which sends timing signals 106 and 108 respectively for recording and for erasing to an information recording and/or erasing device 6 and sends a timing signal 107 for reproducing to the information reproducing device 7, on the basis of the synchronization detecting signal 105.
In conventional methods of timing control, however, since the timing control has been performed only on the basis of the synchronization detecting signal 105, when some error has taken place on the synchronization detecting signal, the timing control can not be correctly performed, thus the reliability in recording, reproducing or erasing is lost.